As part of measures against global warming, measures to reduce emission of carbon dioxide, which is believed to be a major contributor to global warming, are demanded. As biomass grows by photosynthetic fixation of carbon dioxide, it has various proposed applications as a major candidate which does not increase carbon dioxide emission.
Above all, establishment of an effective method for producing ethanol, which is an important chemical expected to serve as an automotive fuel, from biomass is desired.
As a method for producing ethanol from biomass, a method for synthesis of ethanol directly from biomass using a biological process such as fermentation has been proposed.
However, the pretreatment to convert lignin and cellulose contained in woody biomass in an amount of approximately 30% by mass into ethanol has technical and economical problems because it requires a number of steps and a high cost. Another problem is that the rate of utilization of biomass is so low that a large amount of biomass residue is generated.
On the other hand, methods for the production of a gas composed primarily of hydrogen, carbon monoxide, lower hydrocarbons such as methane and ethane, and carbon dioxide from biomass by thermochemical gasification have been proposed (refer to Patent Document 1, for example). In the following, a gas generated by thermochemical gasification of biomass is referred to as “biomass gas.”
In a thermochemical gasification reaction of biomass, a gasification furnace having a fixed bed or fluidized bed is usually used to generate a gas mixture of carbon monoxide and hydrogen. However, the problem is that the gas mixture contains such a small proportion of hydrogen depending on the generation conditions that it is not suitable as a raw material for efficient synthesis of ethanol when used for direct synthesis of ethanol.